1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus, such as a copier, a facsimile machine, a printer, or a multifunction product, that includes the optical scanning device.
2. Description of the Related Art
In accordance with the need for higher speed and higher resolution in the field of image forming apparatuses, such as copiers, facsimile machines, printers, and multifunction productions (MFPs), the number of scanning beams used in optical scanning systems has increased more and more, and optical scanning devices that emit a plurality of beams have become widely used.
In a conventional optical scanning device that scans a target surface with two laser beams (see, for example, Japanese Patent Application Laid-open No. 2007-133334), the amount of jitter in the sub-scanning direction at one edge of the scanning area of the target surface is adjusted to be substantially equal to the amount of jitter at the other edge by setting each laser beam in such a manner that, when the laser beam strikes the center of the scanning area, the chief ray of the laser beam intersects with a point, on a reflecting surface, that is shifted a predetermined distance shifted away from the center of the reflecting surface in the main-scanning direction, with the shift being in a direction away from the light source unit.
Another optical scanning device (see, for example, Japanese Patent Application Laid-open No. 2001-281584) includes a first optical system that converts the state of a beam of light emitted from a light source unit; a second optical system that causes the beam of light to form an image in the shape of a line running in the main-scanning direction; a deflecting unit that has a deflecting surface and reflects, using the deflecting surface, the projected beam of light thereto in the main-scanning direction; and a third optical system that causes the beam of light to form an image on a surface to be scanned. A first state is assumed to be the state where the beam of light goes, after being reflected by the deflecting unit, toward a first end of the surface to be scanned, in which the first end is farther from the light source unit than the optical axis of the third optical system. A second state is assumed to be the state where the beam of light goes toward a second end of the surface to be scanned, in which the second end is closer to the light source unit than the optical axis of the third optical system. A cross-point is assumed to be the intersection of the deflecting surface in the first state and the deflecting surface in the second state. The deflecting unit or the light source unit is arranged so that, after the beam of light passes through the second optical system, the chief ray of the beam does not pass through the cross-point, which suppresses unevenness of the pitch of the scanning lines caused by any optical face tangle error.
Another optical scanning device (see, for example, Japanese Patent Application Laid-open No. 2004-020692) includes a light source unit that emits a plurality of beams of light; a rotary polygon mirror that deflects the two or more beams of light; and an optical scanning system that guides the tow or more beams of light onto a surface to be scanned so that the surface is scanned in the main-scanning direction with the individual beams of light, which are separated in the sub-scanning direction. The surface to be scanned has a rotating axis that is nearly in the same direction as the main scanning direction, but arranged being inclined. One beam of light on the obtuse-angle side out of a plurality of beams is assumed to a beam A, and another beam of light on the acute-angle side is assumed to a beam B, and both the angle of incidence θa of the beam A with the rotary polygon mirror and the angle of incidence θb of the beam B with the rotary polygon mirror are adjusted appropriately so as to suppress jitter caused by the beams of light that strike the surface. The optical scanning device can thus scan the target surface with the beams of light with a high accuracy so as to allow high-speed and high-quality image formation.
Another optical scanning device (see, for example, Japanese Patent Application Laid-open No. 2004-354500) includes a deflecting unit made of a rotary polygon mirror, deflects a beam of light emitted from a light source unit, and also includes an optical scanning system that guides the beam of light deflected by the deflecting unit onto a surface to be scanned. In the optical scanning device, certain parameters satisfy a predetermined relation so as to reduce unevenness of the pitch caused by any optical face tangle error and allow highly fine images to be formed. These parameters include a diameter of the circumscribed circle of the rotary polygon mirror, a number of deflecting surfaces of the rotary polygon mirror, an angle of incidence of the beam of light with the deflecting surface when the center of an effective scanning area is scanned, a maximum swing angle of the deflecting surface while scanning the effective scanning area, and a magnification in a sub-scanning cross section of the optical scanning system.
However, with the above-described conventional optical scanning devices, when a target surface is scanned with a plurality of beams, misalignment (image misalignment) occurs between the beams at the scanning-start-side edge of the target surface and those at the scanning-end-side edge and it is not possible to prevent this misalignment, which causes degradation to the formed image.
The misalignment and the degradation of the formed image due to the misalignment are described in detail below with reference to FIGS. 10 and 11.
FIG. 10 is a schematic diagram of the configuration of a conventional optical scanning device (scanning optical device) that uses a plurality of beams. The optical scanning device includes a light source 200 having three luminous points 201, 202, and 203.
The reference numerals B1, B3, and B5 shown in FIG. 10 denote the chief rays of the beams emitted from the luminous points 201, 202, and 203, respectively.
The reference numerals 204 and 205 denote beam shaping lenses. The reference numeral 205 denotes a cylinder lens having a power only in a direction perpendicular to the deflecting/scanning direction (hereinafter, “sub-scanning direction”) and causes each beam of light to form an image in the shape of a line running in the deflecting/scanning direction.
The reference numeral 206 denotes a rotary polygon mirror; the reference numeral 207 denotes a scanning lens; and the reference numeral 208 denotes a surface to be scanned.
In this conventional optical scanning device that uses a plurality of beams, each beam strikes a deflecting surface 206a of the rotary polygon mirror 206 within a deflecting/scanning surface at a different angle.
As described above, the deflecting surface 206a of the rotary polygon mirror 206 receives each beam at a different rotation angle when a predetermined position on the surface 208 is scanned (or when each beam enters the scanning lens at a predetermined viewing angle); therefore, if various parameters are not adjusted appropriately, the image forming positions within the scanning area become asymmetric in the sub-scanning direction (i.e. ε≠ε′), as shown in FIG. 11, i.e., a misalignment (also called “asymmetric misalignment”) occurs. A large degree of unbalance of such a misalignment may cause degradation of the formed image.